In the field of electronic devices, it is often necessary for a current to be measured, for example in order to ensure that a sufficient current is provided to a load. Power over Ethernet (PoE) is an example of where a minimum current is required to be provided over an Ethernet connection. PoE is a system for transferring electrical power, along with data, to remote devices over, for example, a standard twisted pair cable in an Ethernet network. Such a system is useful for power IP (Internet Protocol) telephones, wireless LAN (Local Area Network) access points, network cameras, etc. There are several PoE implementations, including many ad-hoc techniques. However, the most common implementation of PoE is that defined in IEEE 802.3af. As will be appreciated, a load as perceived by a PoE driver will vary depending on, for example, the remote device(s) connected thereto, as well as the length of cable, etc. Accordingly, PoE drivers are required to be able to adapt to the varying loads that they might encounter in order to ensure that a sufficient current is provided.
PoE drivers are typically ‘low side’ (i.e. operably coupled between a load and ground), and are typically required to convey a current ranging from, say, 5 mA up to 1 A to a remote device. Existing approaches for ensuring that a sufficient current is being provided are typically based on the use of a series sense resistor, whereby the voltage across the sense resistor provides an indication as to the current flowing there through. The output of a PoE driver may then be adjusted in response to the voltage monitored across the sense resistor in order to ensure that the required current is flowing.
The value of the sense resistor is initially selected based on a trade-off between accuracy and power dissipation; the higher the value of the sense resistor the greater the accuracy that can be achieved, but also the greater the power dissipation, and vice versa. For example, take the case where a 5 mA current is to be measured with 10% accuracy. A 1 Ohm sense resistor will result in a sense voltage of 5 mV, making a 10% accuracy (500 μV) achievable with trimmed detectors. However, the sense resistor will dissipate 1 W of power with a 1 A current, which would be duplicated for each port. Clearly such high power dissipation is undesirable. A 0.2 Ohm sense resistor would result in only a 200 mW power dissipation with a 1 A current. However, a 0.2 Ohm sense resistor would result in only a 1 mV sense voltage with a 5 mA current, requiring a 10% accuracy corresponding to a 100 μV precision which is difficult to achieve.